1. Field of the Invention
The present invention relates to an ink jet printing apparatus which performs printing by ejecting ink from a print head to a print medium.
2. Description of the Related Art
It is known that the ink adheres to a print head surface (hereinafter referred to as an ejection face) where ejection openings (nozzles) are formed in an ink jet printing apparatus in general. This is because when the ink is ejected from a nozzle, a large amount of small ink droplets (hereinafter referred to as mist) are ejected separately from the primary ink droplets (main droplets) for printing purposes. The mist generated in a large amount drifts in a space between the print head and a print medium and comes back and adheres to the ejection face of the print head due to its small mass. Moreover, there is also mist which is formed by a part of main droplets once landed on the print medium surface is bounced back and re-adhering the ejection face.
Such mist let the ejection face wet and especially when adhered to a vicinity of the ejection opening, this adhered ink causes a defective ejection such as a reduction in ink ejecting directivity (this phenomenon is called ‘deflection’ since the ejecting direction is deflected by pulling ejected main droplets. This may then cause a reduction in printing image qualities. In addition, it may also cause ejection failures in an extreme case.
In order to resolve such a state where the ejection face gets wet and may cause defective ejection or ejection failure), a wiping member to wipe the ejection face called a wiper is generally installed in the ink jet printing apparatus. An action to wipe away (wiping) the ink mist adhered to the ejection face is carried out at an appropriate timing. As an example for such an action, Japanese Patent Application Laid-open No. 07-125228(1995) discloses a method to determine the timing of wiping by combining the use of a timer and counting (dot count) of ink ejection number by the print head. In addition, Japanese Patent Application Laid-open No. 2001-121717 discloses a method for determining the timing of wiping by combining usual dot count and printing duty.
Moreover, there is a case where the mist adhered to the ejection face increases its viscosity due to the evaporation of ink solvents, by a high temperature of the print head or an inability to carry out wiping during long time-requiring printing. For these reasons, in a case where deterioration in wiping ability is a concern, there are those adopting responses like carrying out wiping after wetting a wiper in advance with ink or stock/prepared solutions of other solvents (for example, Japanese Patent Application Laid-open No. 2002-166560).
However, there is a case where favorable wiping ability (ejection face cleaning ability) cannot be displayed due to the low efficiency in wiping even when the wiper is provided as described above and the timing of the wiping is determined. For example, a plurality of ejecting portions capable of ejecting ink with different color tones (including color and concentration) in the main scanning direction may be juxtaposed in an ink jet print head adopted in the ink jet printing apparatus of a serial scan type. For such a configuration, a wiper carrying out the wiping operation all at once is provided in a plurality of ejecting portions in many cases.
A case where there is an extreme differences in properties of ink used among the ejecting portions is considered in such a configuration. For example, when there are 6 ejecting portions a to f present corresponding to 6 colours of ink A to F and especially when the ink C ejected from the ejecting portion c generates mist readily, much mist is adhered to the ejecting face of the ejecting portion c than to those of others even when printing with similar duty is carried out in each ejecting portion.
Here, a difference in the amount of ink mist generated is dominated by various factors. As factors due to characteristics of ink alone, examples include viscosity, surface tension, and contact angle with a material forming the nozzle. Moreover, the amount of mist generated is also further changed in a variety of ways by the combination of power applied to make the print head or the ejecting portion carry out the ejection action and physical properties of ink. Furthermore, the differences in the amount of ink mist generated and the amount of ink adhered between each ejecting portion occur when considering various factors such as ejection speed, ink landing stability onto the print medium, recovery ability from fixed adhesion of the ink, and the prevention of ink solvent evaporation from the nozzle at the time of printing, in order to take advantage of characteristics of each ink.
Therefore, to carry out the wiping operation in the identical conditions against a plurality of ejecting portions is not necessarily favourable since the number of wiping will be too small for one ejecting portion (for example, above described ejecting portion A) and there is a concern that the cleaning of the ejecting portion will be insufficient. However, these differences in the amount of ink mist generated or the adhered amount have not been hitherto considered. For example, even when the dot count was carried out to determine the timing of the wiping operation by considering the printing duty as described in Japanese Patent Application Laid-open No. 2001-121717, the occurrence of variations in the amount of mist in each ejecting portion could not be dealt with.
Moreover, an increase in viscosity due to evaporation of ink solvents has not hitherto considered and it has been thought that all inks exhibit the identical characteristics including evaporation and phenomena associated with it. Consequently, although differences exist in the ease of wiping due to the difference in evaporation characteristic of each ink when the wiping operation is actually carried out, the wiping operation has been carried out against a plurality of ejecting portions all at once and uniformly. Accordingly, after such the wiping, an ejection face of some ejecting portion becomes a good state, while an ejection face of some ejecting portion is still unsatisfactory state.